This invention relates to a magnetic recording and reproducing apparatus such as a video tape recorder of helical scan type in which a plurality of rotary magnetic heads are used to sequentially record an input signal on a magnetic recording medium such as a magnetic tape.
FIG. 3 shows a signal recording system incorporated in a prior art magnetic recording and reproducing apparatus of this type, for example, a VHS system VTR.
Referring to FIG. 3, an input video signal a is passed through a low-pass filter (abbreviated hereinafter as an LPF) 1 and a band-pass filter (abbreviated hereinafter as a BPF) 2 to be separated into a luminance signal b and a color signal c respectively. After the luminance signal b is subjected to frequency modulation in an FM modulation circuit 3, and the color signal C is converted into a down converted chrominance signal in a down conversion circuit 6, these signals b and c are passed through level adjusters 4 and 7 respectively and then mixed together in a mixer circuit 5. The mixture of the FM luminance signal and the down converted chrominance signal is applied to a recording amplifier circuit 8 to be suitably amplified, and the output signal of the recording amplifier circuit 8 is applied to a pair of rotary magnetic heads 9 and 10 through a rotary transformer 11 to be alternately recorded on respective video tracks on a magnetic recording medium (not shown). Resistors 13 and 14 are connected in series with the primary windings of the respective channels in the rotary transformer 11, and a monitor output for controlling the level adjusters 4 and 7 is derived from a terminal 12 connected to the resistor 13.
The recording and reproducing characteristic of a magnetic recording and reproducing apparatus of this kind is described, for example, at page 49 of a book entitled "VTR" written and edited by Kenichi Sawazaki and published by the Corona Publishing Co. in Japan in October, 1971. FIG. 4 shows the recording and reproducing characteristic described in the above book. It will be apparent from FIG. 4 that the level of a reproduced output changes according to the level of a recording current supplied to magnetic heads.
Therefore, in the prior art magnetic recording and reproducing apparatus shown in FIG. 3, the recording current levels of the FM luminance signal and the down converted chrominance signal are adjusted by the respective level adjusters 4 and 7 while monitoring the values of the recording currents actually supplied to the rotary magnetic heads 9 and 10 by observing the monitor output derived from the terminal 12. Thus, the recording current levels of the FM luminance signal and the down converted chrominance signal can be adjusted so as to be optimized regardless of fluctuation of the gains of the FM modulation circuit 3, down conversion circuit 6, mixer circuit 5 and recording amplifier circuit 8 and also regardless of fluctuation of the level of the input video signal a itself. The optimum value of the recording current of the FM luminance signal is such that the level of the reproduced output becomes maximum so as to provide a sufficient S/N ratio. However, as will be apparent from FIG. 4, this optimum value differs depending on the frequency of the recording signal. Therefore, it is a common practice that the optimum value is so selected as to provide a maximum reproduced output level with respect to the center frequency (3 to 4 MHz) of the carrier. In the case of the down converted chrominance signal, a problem, for example, diamond cross beat interference due to cross modulation with the FM luminance signal arises when the level adjuster 7 adjusts the level of the low frequency band color signal so as to maximize the reproduced output level. Therefore, the level of the low frequency band color signal is commonly adjusted to be lower by 10 dB or less than that of the FM luminance signal.
In the prior art magnetic recording and reproducing apparatus described above, the level adjusters 4 and 7 must be manually adjusted while observing the monitor output derived through the terminal 12. Thus, the adjustment is troublesome and time consuming. Also, because variable resistors are required as these level adjusters 4 and 7, the entire recording circuit cannot be integrated into an IC, and an increase in the costs of the circuit parts is inevitable. Further, the fact that these variable resistors are incorporated as externally mounted parts leads to the problem that the manufacturing process becomes correspondingly complex.
Further, because the recording current levels are adjusted in the recording circuit system common to the two rotary magnetic heads 9 and 10, the values of the recording currents supplied to these rotary magnetic heads 9 and 10 cannot be adjusted independently of each other. In the case of the prior art apparatus shown in FIG. 3, the value of the recording current supplied to the rotary magnetic head 9 only (the voltage drop across the resistor 13 only) is monitored so as to adjust the level adjusters 4 and 7. Therefore, such a problem arises that the value of the recording current supplied to the other rotary magnetic head 10 is not necessarily adjusted to its optimum value due to, for example, a difference between the inductances of the rotary magnetic heads 9 and 10.